In motor vehicle engineering, “brake-by-wire” brake systems are becoming ever more widespread. Such brake systems often comprise not only a master brake cylinder which can be activated by the vehicle driver but also an electrically controllable pressure supply device by means of which the wheel brakes or the master brake cylinder is activated in the “brake-by-wire” operating mode. In order to give the vehicle driver a pleasant pedal sensation in the “brake-by-wire” operating mode, the brake systems usually comprise a brake pedal sensation simulation device. In these brake systems, the wheel brakes can also be activated on the basis of electronic signals without active participation by the vehicle driver. These electronic signals can be output, for example, by an electronic stability program or an adaptive cruise control system.
International patent application WO 2010/091883 A1, which is incorporated by reference, discloses a brake system with a brake booster whose master brake cylinder is driven mechanically by an electric motor. The master brake cylinder is hydraulically connected to four wheel brakes, wherein the wheel brakes can be disconnected from the master brake cylinder by means of a switching valve in each case. In order to operate the brake system, knowledge of the pressure/volume characteristic curve of the individual wheel brakes is important. In accordance with WO 2010/091883 A1, the pressure/volume characteristic curve is recorded individually for each wheel brake when the vehicle is in a stationary state, by detecting the volume given knowledge of the master brake cylinder pressure over the corresponding piston travel of the master brake cylinder. It is felt to be disadvantageous here that this method can be carried out only to a chronologically limited extent, specifically when the vehicle is stationary. Furthermore, the execution of this method is felt to be disadvantageous both when the brake pedal is activated and when the brake pedal is unactivated. In the case of an activated brake pedal, it is, under certain circumstances, impossible to obtain a sufficient braking effect via one wheel to stop the vehicle reliably. In the event of the brake system being currently serviced, the servicing personnel could be injured by the clamping force at the wheel brake in the case of a brake pedal which is unactivated. The method which is described in WO 2010/091883 A1 should therefore not be carried out during ongoing operation but only during servicing, with separate triggering.